State of the Art
Typically, integrated circuits are manufactured by the deposition of layers of predetermined materials to form the desired circuit components on a silicon wafer semiconductor substrate. As the layers are deposited on the substrate wafer to form the desired circuit component, the planarity of each of the layers is an important consideration because the deposition of each layer produces a rough, or nonplanar, topography initially on the surface of the wafer substrate and, subsequently, on any previously deposited layer of material.
Typically, photolithographic processes are used to form the desired circuit components on the wafer substrate. When such photolithographic processes are pushed to their technological limits of circuit formation, the surface on which the processes are used must be as planar as possible to ensure success in circuit formation. This results from the requirement that the electromagnetic radiation used to create a mask, which is used in the formation of the circuits of the semiconductor devices in wafer form, must be accurately focused at a single level, resulting in the precise imaging over the entire surface of the wafer. If the wafer surface is not sufficiently planar, the resulting mask will be poorly defined, causing, in turn, a poorly defined circuit which may malfunction. Since several different masks are used to form the different layers of circuits of the semiconductor devices on the substrate wafer, any non-planar areas of the wafer will be subsequently magnified in later deposited layers.
After layer formation on the wafer substrate, either a chemical etch-back process of planarization, or a global press planarization process typically followed by a chemical etch-bach process of planarization, or chemical mechanical planarization process may be used to planarize the layers before the subsequent deposition of a layer of material thereover. In this manner, the surface irregularities of a layer may be minimized so that subsequent layers deposited thereon do not substantially reflect the irregularities of the underlying layer.
One type of chemical etch-back process of planarization, illustrated in EUROPEAN PATENT APPLICATION 0 683 511 A2, uses a coating technique in which an object having a flat surface is used to planarize a coating material applied to the wafer surface prior to a plasma reactive ion etching process being used to planarize the wafer surface. Often, however, the planarization surface will contain defects, such as pits or other surface irregularities. These may result from defects in the flat surface used for planarizing or from foreign material adhering to the flat surface. The etching of such a wafer surface having irregularities will, at best, translate those undesirable irregularities to the etched surface. Further, since some etching processes may not be fully anisotropic, etching such irregular surfaces may increase the size of the defects in the etched wafer surface.
One type of global press planarization process, illustrated in U.S. Pat. No. 5,434,107, subjects a wafer with features formed thereon having been coated with an inter-level dielectric material to an elevated temperature while an elevated pressure is applied to the wafer using a press until the temperature and pressure conditions exceed the yield stress of the upper film on the wafer so that the film will attempt to be displaced into and fill both the microscopic and local depressions in the wafer surface. It should be noted that the film is only deformed locally on the wafer, not globally, during the application of elevated temperature and pressure since the object contacting the surface of the wafer will only contact the highest points or areas on the surface of the wafer to deform or displace such points or areas of material on the entire wafer surface. Other non-local depressions existing in the wafer are not affected by the pressing as sufficient material is not displaced thereinto. Subsequently, the temperature and pressure are reduced so that the film will become firm again thereby leaving localized areas having a partially planar upper surface on portions of the wafer while other portions of the wafer surface will remain non-planar.
In one instance, global planar surfaces are created on a semiconductor wafer using a press located in a chamber. Referring to drawing FIG. 1, a global planarization apparatus 100 is illustrated. The global planarization apparatus 100 serves to press the surface of a semiconductor wafer 120 having multiple layers including a deformable outermost layer 122 against a fixed pressing surface 132. The surface of the deformable layer 122 will assume the shape and surface characteristics of the pressing surface 132 under the application of force to the wafer 120. The global planarization apparatus 100 includes a fully enclosed apparatus having a hollow cylindrical chamber body 112 having open top and bottom ends 113 and 114, respectively, and interior surface 116 and an evacuation port 111. A base plate 118 having an inner surface 117 is attached to the bottom end 114 of chamber body 112 by bolts 194. A press plate 130 is removably mounted to the top end 113 of chamber body 112 with pressing surface 132 facing base plate 118. The interior surface 116 of chamber body 112, the pressing surface 132 of press plate 130 and the inner surface 117 of base plate 118 define a sealable chamber. Evacuation port 111 can be positioned through any surface, such as through base plate 118, and not solely through chamber body 112.
The press plate 130 has a pressing surface 132 with dimensions greater than that of wafer 120 and being thick enough to withstand applied pressure. Press plate 130 is formed from non-adhering material capable of being highly polished so that pressing surface 132 will impart the desired smooth and flat surface quality to the surface of the deformable layer 122 on wafer 120. Preferably, the press plate is a disc shaped quartz optical flat.
A rigid plate 150 having top and bottom surfaces 152 and 154, respectively, and lift pin penetrations 156 therethrough is disposed within chamber body 112 with the top surface 152 substantially parallel to and facing the pressing surface 132. The rigid plate 150 is constructed of rigid material to transfer a load under an applied force with minimal deformation.
A uniform force is applied to the bottom surface 154 of rigid plate 150 through the use of a bellows arrangement 140 and relatively pressurized gas to drive rigid plate 150 toward pressing surface 132. Relative pressure can be achieved by supplying gas under pressure or, if the chamber body 112 is under vacuum, allowing atmospheric pressure into bellows 140 to drive the same. The bellows 140 is attached at one end to the bottom surface 154 of rigid plate 150 and to the inner surface 117 of base plate 118 with a bolted mounting plate 115 to form a pressure containment that is relatively pressurized through port 119 in base plate 118. One or more brackets 142 are mounted to the inner surface 117 of the base plate 118 to limit the motion toward base plate 118 of the rigid plate 150 when bellows 140 is not relatively pressurized. The application of force through the use of a relatively pressurized gas ensures the uniform application of force to the bottom surface 154 of rigid plate 150. The use of rigid plate 150 will serve to propagate the uniform pressure field with minimal distortion. Alternately, the bellows 140 can be replaced by any suitable means for delivering a uniform force, such as a hydraulic means.
A flexible pressing member 160 is provided having upper and lower surfaces 162 and 164, respectively, which are substantially parallel to the top surface 152 of rigid plate 150 and pressing surface 132. Lift pin penetrations 166 are provided through member 160. The flexible member 160 is positioned with its lower surface 164 in contact with the top surface 152 of rigid plate 150 and lift pin penetrations 166 aligned with lift penetrations 156 in rigid plate 150. The upper surface 162 of the member 160 is formed from a material having a low viscosity that will deform under an applied force to close lift pin penetrations 166 and uniformly distribute the applied force to the wafer, even when the top surface 152, the upper surface 162 and/or the lower surface 164 is not completely parallel to the pressing surface 132 or when thickness variations exist in the wafer 120, rigid plate 150 or member 160, as well as any other source of non-uniform applied force.
Lift pins 170 are slidably disposable through lift pin penetrations 156 and 166, respectively, in the form of apertures, to contact the bottom surface 126 of wafer 120 for lifting the wafer 120 off the top surface 162 of member 160. Movement of the lift pins 170 is controlled by lift pin drive assembly 172, which is mounted on the inner surface 117 of the base plate 118. The lift pin drive assembly provides control of the lift pins 170 through conventional means. Lift pins 170 and lift pin drive assembly 172 are preferably positioned outside the pressure boundary defined by the bellows 140 to minimize the number of pressure boundary penetrations. However, they can be located within the pressure boundary, if desired, in a suitable manner.
A multi-piece assembly consisting of lower lid 180, middle lid 182, top lid 184, gasket 186 and top clamp ring 188 are used to secure the press plate 130 to the top end 113 of chamber body 112. The ring-shaped lower lid 180 is mounted to the top end 113 of chamber body 112 and has a portion with an inner ring dimension smaller than press plate 130 so that press plate 130 is seated on lower lid 180. Middle lid 182 and top lid 184 are ring-shaped members having an inner ring dimension greater than press plate 130 and are disposed around press plate 130. Middle lid 182 is located between lower lid 180 and top lid 184. A gasket 186 and top clamp ring 188 are members having an inner ring dimension less than that of press plate 130 and are seated on the surface of press plate 130 external to the chamber. Bolts 194 secure press plate 130 to the chamber body 112.
Heating elements 190 and thermocouples 192 control the temperature of the member 160.
In operation, the top clamp ring 188, gasket 186, top lid 84, and middle lid 82 are removed from the body 112 and the press plate 130 lifted from lower lid 180. The bellows 140 is deflated and rigid plate 150 is seated on stand off brackets 142. The wafer 120 is placed on the flexible member 160 with the side of the wafer 120 opposite the deformable layer 122 in contact with flexible member 160. The press plate 130 is mounted on the lower lid 180 and the middle lid 182, and upper lid 184 are installed and tightened using gasket 186 and top clamp ring 188 sealing press plate 130 between top clamp ring 188 and lower lid 180. The temperature of member 160, press plate 130, and rigid plate 150 are adjusted through the use of heating elements 190 monitored by thermocouples 192 to vary the deformation characteristics of the layer 122 of wafer 120. Chamber body 112 is evacuated through port 119 to a desired pressure.
A pressure differential is established between the interior and exterior of the bellows 140, whether by pressurizing or by venting when the chamber body 112 having been evacuated thereby drives rigid plate 150, member 160, and wafer 120 toward press plate 130 and brings deformable layer 122 of wafer 120 into engagement with pressing surface 132 of press plate 130. Upon engagement of wafer 120 with press plate 130, the continued application of force will deform the flexible member 160 which, in turn, serves to close lift penetrations 166 and distribute the force to ensure the wafer 120 experiences uniform pressure on its surface 122. After the wafer 120 has been in engagement with pressing surface 132 for a sufficient time to cause surface 122 to globally correspond to the pressing surface layer 132, the surface 122 is hardened or cured. The pressure is released from the bellows 140, thereby retracting wafer 120, member 160, and rigid plate 150 from the press plate 130. The downward movement of rigid plate 150 will be terminated by its engagement with stand off offset brackets 142.
Once the rigid plate 150 is fully retracted, the vacuum is released in chamber body 112. Lift pins 170 are moved through lift penetrations 156 in the rigid plate 150 and lift penetrations 166 in the member 160 to lift wafer 120 off the member 160. The top clamping ring 188, gasket 186, top lid 184, middle lid 182, and press plate 130 are removed and the wafer 120 is removed off lift pins 170 for further processing.
Once the wafer is removed, it will be subjected to an etch to establish the planar surface at the desired depth. A system used or depicted in FIG. 1 provides an optimal method of deforming a flowable, curable material to form a generally planarized surface. However, the method is still subject to yielding a wafer surface with irregularities therein, and the need for the subsequent etch to define the desired surface height will still result in undesirable transfer and possible enlargement of any such surface irregularities.
Conventional chemical mechanical planarization processes are used to planarize layers formed on wafer substrates in the manufacture of integrated circuit semiconductor devices. Typically, a chemical mechanical planarization (CMP) process planarizes a non-planar irregular surface of a wafer by pressing the wafer against a moving polishing surface that is welted with a chemically reactive, abrasive slurry. The slurry is usually either basic or acidic and generally contains alumina or silica abrasive particles. The polishing surface is usually a planar pad made of a relatively soft, porous material, such as a blown polyurethane, mounted on a planar platen.
Referring to drawing FIG. 2, a conventional chemical mechanical planarization apparatus is schematically illustrated. A semiconductor wafer 112 is held by a wafer carrier 111. A soft, resilient pad 113 is positioned between the wafer carrier 111 and the wafer 112. The wafer 112 is held against the pad 113 by a partial vacuum. The wafer carrier 111 is continuously rotated by a drive motor 114 and is also designed for transverse movement as indicated by the arrows 115. The rotational and transverse movement is intended to reduce variability in material removal rates over the surface of the wafer 112. The apparatus further comprises a rotating platen 116 on which is mounted a polishing pad 117. The platen 116 is relatively large in comparison to the wafer 112, so that during the chemical mechanical planarization process, the wafer 112 may be moved across the surface of the polishing pad 117 by the wafer carrier 111. A polishing slurry containing a chemically reactive solution, in which abrasive particles are suspended, is delivered through a supply tube 121 onto the surface of the polishing pad 117.
Referring to drawing FIG. 3 a typical polishing table is illustrated in top view. The surface of the polishing table 1 is precision machined to be flat and may have a polishing pad affixed thereto. The surface of the table rotates the polishing pad past one or more wafers 3 to be polished. The wafer 3 is held by a wafer holder, as illustrated hereinbefore, which exerts vertical pressure on the wafer against the polishing pad. The wafer holder may also rotate and/or orbit the wafer on the table during wafer polishing.
Alternately, the table 1 may be stationary and serve as a supporting surface for individual polishing platens 2, each having their own individual polishing pad. As illustrated in U.S. Pat. No. 5,232,875, each platen may have its own mechanism for rotating or orbiting the platen 2. A wafer holder will bring a wafer in contact with the platen 2 and an internal or external mechanism to the wafer holder may be used to also rotate the wafer during the polishing operation. In a polishing table having multiple individual platens, each platen must be precision machined.
The wafers 3 are typically stored and transported in wafer cassettes which hold multiple wafers. The wafers 3 or wafer holders are transported between the wafer cassettes and the polishing table 1 using the wafer transport arm 4. The wafer transport arm 4 will transport the wafers 3 between the polishing table and the stations 5, which may be wafer cassette stations or wafer monitoring stations.
The polishing characteristics of the polishing pad will change during use as multiple wafers 3 are polished. The glazing or changing of the polishing characteristics will affect the planarization of the surface of the wafers 3 if the pads are not periodically conditioned and unglazed. The pad conditioner 6 is used to periodically unglaze the surface of the polishing pad. The pad conditioner 6 has a range of motion which allows it to come in contact with the individual pads and conduct the periodic unglazing and then to move to its rest position.
The pressure between the surface of the wafer to be polished and the moving polishing pad may be generated by either the force of gravity acting on the wafer and the wafer carrier or by mechanical force applied normal to the wafer surface. The slurry may be delivered or injected through the polishing pad onto its surface. The planar platens may be moved in a plane parallel to the pad surface with either an orbital, fixed-direction vibratory or random direction vibratory motion.
While a chemical mechanical planarization process is an effective process to planarize the surface of a wafer, variations in height on the surface to be planarized by the chemical mechanical planarization process, although minimized through the chemical mechanical planarization process, will often not be completely removed to yield an optimally planar surface. As is well known in the art, the chemical mechanical planarization process polishing pad will deform, or "dish", into recesses between structures of the surface of the wafer. The structure spacing on the wafer which will yield this "dishing" is clearly a function of various factors, such as the pad composition, the polishing pressure, etc. This pad "dishing" will clearly lead to less than optimal planarization of the surface of the wafer. Further, the surface irregularities extending into or down to the wafer surface being planarized tend to collect slurry, thereby causing such areas of the wafer to be subjected to the corrosive effects of the slurry longer than other areas of the wafer surface which do not collect the slurry.
To help minimize polishing pad deformation (dishing) caused by surface irregularities formed by the integrated circuit components on the wafer surface, dummy structures have also been included on the wafer surface in an attempt to provide a more uniform spacing of structures on the wafer surface. While the use of such dummy structures will often be useful, the ultimate result is also highly dependent upon the later chemical mechanical planarization process conditions.
Therefore, a need exists to reduce the surface irregularities on a wafer before the chemical mechanical planarization process to facilitate planarization of the wafer surface irregularities by such process and to facilitate planarization which provides greater latitude in the chemical mechanical planarization process parameters.